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What is transOgg?

For a long time there have been discussions of what we in Xiph would change in the Ogg container once we considered it appropriate to break spec. transOgg is an updated Ogg container (ie Ogg v 2) that makes some changes to the
Ogg transport layer and more directly tackles metadata. transOgg: Changes from Ogg summarizes the major changes from the original Ogg container design.
This page presents an overview of nebulous transOgg design points and rationale.

As of today, transOgg exists only in the form of the whitepapers and structure proposals here. This spec is only in the very early stages of being written. No code exists as yet.

Design Points

In no particular order:

transOgg is degined for local storage and packet-based transport. Some intended uses include:

transOgg is designed for variable, unpredictably sized data payloads with no minumum or maximum size.

The transOgg container is structurally designed for streaming (both live and progressive download). It is not possible to construct a valid transOgg stream that is unsuitable for streaming.

transOgg defines two steam types: continuous-time and discontinuous-time. Continuous-time streams are gapless media such as video and audio. Discontinuous-time streams are media types with unpredictably or irregularly placed data, such as subtitles and timed metadata.

transOgg metadata is structurally encapsulated into the transport stream but located at fixed, predictable positions (excepting streamed metadata, which are treated as a discontinuous stream).

A transOgg stream always captures and begins demux within 128kB maximum. Fine-grained capture is necessary for efficient streaming, seeking and scrubbing. The overhead tradeoff of a frequent capture pattern is negligable and fully offset by other improvements.

Multiplexing of multiple elementary streams is performed by interleaving at the page level. The multiplexing algorithm is fully specified, deterministic and delivers optimal buffering behavior. There is no educated guessing or multiple possible practices.

transOgg buffering is simple and explicitly specified.

transOgg implements nonlinear features such as menus, chapters, loop points, and branch points out of its linear stream transport by borrowing from CMML, Skeleton and Matroska's EBML metadata specification.

Valid transOgg streams may be concatenated to form a new, valid transOgg stream. Mandatory reverse linkage at the end of each stream eliminates the need for interpolated bisection search when opening concatenated streams. Cross-link metadata provides file-global indexing and chaptering for chained streams.

transOgg metadata begins and ends every stream. Metadata is mandatory, fully specified, and part of 'container knowledge'.

A transOgg stream must begin with the master metadata header. This master header is the first page[s] of the physical transOgg bitstream as well as the logical master metadata stream.

The metadata stream is a discontinuous stream that may provide additional timed metadata and events throughout the stream, similar to NUT 'info packets'.

A transOgg stream must end with metadata footer page[s] that provide, among other things, reverse linkage to the beginning of the stream.

Tags (user contributed metadata) and Cues (the index) may appear at the head of the stream or in the footer, as may any other metadata elements that could not be known before stream end in a live stream (eg, duration). Single-pass creation tools write these elements in the footer metadata. Tools can later move these elements to the header metadata. All other metadata elements may appear only in the header.

Headerless capture, multicast, and stateless unicast MAY be supported within the metadata stream using "rolling headers", similar to the "rolling intra" mechanism proposed for the Theora videocodec. This allows stream capture and playback in a bounded timeperiod without OOB transmission of headers or bitrate spikes. It also facilitates file recovery in the event the stream headers are lost.

Structural codec metadata, such as timebase, keyframing, coding delay, page duration, etc, are replicated in the transOgg container. Unlike Ogg (and to a lesser degree Matroska), no knowledge must be queried or assumed based on the specific codecs in use inorder to mux, demux, remux, repaginate, transmux, or seek in a bitstream.

As in NUT, all streams have their own rational timebase. The encoding used is a parameterized generalization of Ogg granule positions. The granule timebase and parameters are fully specified and declared in the container. The granule mechanism is capable of exact sample positioning without approximation, expressing PTS and DTS of out-of-order encodings, preroll/delay of keyframe-lesscodecs, and distance from last syncpoint.

All encapsulated packets are stamped with full DTS, PTS, duration, delay, and syncpoint distance.

the core transOgg container seeks to avoid optional structures, switches, code paths, and features in its framing mechanisms. Optional structures and features are acceptable (and necessary) within metadata.

High level design

The high level transOgg design consists of a transport, metadata, and
specified practices. These pieces are conceptually seperable, but the
container cannot succeed missing any one.

Transport

Transport is the mechanism of encapsulating and delivering data.
transOgg uses a modified/updated Ogg page mechanism for data and
metadata delivery.

Transport benefits from a simple, fixed encoding. Optional features,
arbitrary extensibility, recursive or non-flat heirarchy, and
conditional semantic encoding are undesirable complications in a low
level transport and should be used only when clearly advantageous or
unavoidable. Specifying transport as a self-contained layer also
seperates correct transport behavior and corner cases from the rest of
the container behavior.

Raw A/V media is fundamentally time-linear in atomic form. Networks
and storage media deliver data for consumption in a time-linear stream
of bytes. Both suggest that a linear encoding is optimal for the
low-level encapsulation. Metadata can build non-linear presentation
from linear segments. Nonlinear structural metadata appears at the
beginning and end of the stream; as such, this metadata can also be
placed in the linear transport easily as the beginning and end of
data. Encapsulating metadata in the transport like the streaming data
also makes it trivial to support streamed metadata and 'rolling
headers' using preexisting transport mechanisms. (*-- discuss both
chaining and multi-segment; metadata that can reach across segments?
etc?)

Metadata

Metadata is everything in a stream/file that is not the media stream
itself. transOgg proposes use a packed encoding for metadata types unlikely to see much flux, and an extensibly-structured encoding for more free-form types (eg, Matroska-style metadata in an EBML
encoding for stream tagging).

Metadata encompasses a number of semantically quite different
concepts, eg:

1: data about how the individual streams are encoded and encapsulated (codec id, timebase, continuous/discontinuous encoding, codec private data, etc). This metadata is essential to base container operation and must function as container knowledge. It is always located in a fixed position at the beginning of the file as it must be read to bootstrap container operation.

2: data about navigating the file as it's currently arranged (linkages, indexing, chapters). This data is either essential to high-level container operation or essential to the application depending on how the implementation abstractions work out.

3: data about how the streams are presented for playback (langauge, primary angle, available soundtrack languages, menus). This data is needed by the application.

4: user-supplied comments, one-shot auxiliary data (tags, album art). This data is needed by the application and the user.

Each kind of metadata shares some basic traits. It is heirarchical,
largely conditional, and benefits from a rich stable of optional
elements to be used as appropraite. It is also likely that aside from
the MUST elements required for playback (mostly from list 1), not all
metadata will be interesting to all players. An obvious use case is a
memory and CPU constrained mobile device with no bitmapped display
which would want to entirely ignore/skip large album art chunks.

Specified practices

Specified practices provide the instruction manual for proper use of
the container system in all cases where the container structure allows
multiple behavioral or encoding possibilities. 'best practices' is a
more common term, however 'best' connotes that such guidelines are
merely suggestive. Specified practices are effectively MUST clauses
that govern proper behavior rather than valid data.

Specified practices are an especially weak point of current FOSS
container offerings. Developers of open projects often value a system
that offers many equivalent ways of performing a task, and leave it to
higher-level developers (or worse, the user) to somehow make an
informed decision of how to proceed. In the container space, this is
undesirable bordering on disaster; 'more' is 'less'.

To choose an absurd example, it might be technologically nifty to be
able to place an index into the middle of a file, but what could it
possibly accomplish? Absurd flexibility results in absurd bugs. When
absurd or clearly suboptimal choices are structurally possible, the
spec should not be silent on the subject. The spec MUST explicitly
address allowed and disallowed behavior to the most complete degree
achievable.

Proposals / Commentary Requested

transOgg is a specification in the early stages of design. As such, we're soliciting feedback on specific design proposals below, with more to be added over time as the design process approaches new milestones.